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-============
-Watch Notify
-============
-
-See librados for the watch/notify interface.
-
-Overview
---------
-The object_info (See osd/osd_types.h) tracks the set of watchers for
-a particular object persistently in the object_info_t::watchers map.
-In order to track notify progress, we also maintain some ephemeral
-structures associated with the ObjectContext.
-
-Each Watch has an associated Watch object (See osd/Watch.h).  The
-ObjectContext for a watched object will have a (strong) reference
-to one Watch object per watch, and each Watch object holds a
-reference to the corresponding ObjectContext.  This circular reference
-is deliberate and is broken when the Watch state is discarded on
-a new peering interval or removed upon timeout expiration or an
-unwatch operation.
-
-A watch tracks the associated connection via a strong
-ConnectionRef Watch::conn.  The associated connection has a
-WatchConState stashed in the OSD::Session for tracking associated
-Watches in order to be able to notify them upon ms_handle_reset()
-(via WatchConState::reset()).
-
-Each Watch object tracks the set of currently un-acked notifies.
-start_notify() on a Watch object adds a reference to a new in-progress
-Notify to the Watch and either:
-
-* if the Watch is *connected*, sends a Notify message to the client
-* if the Watch is *unconnected*, does nothing.
-
-When the Watch becomes connected (in PrimaryLogPG::do_osd_op_effects),
-Notifies are resent to all remaining tracked Notify objects.
-
-Each Notify object tracks the set of un-notified Watchers via
-calls to complete_watcher().  Once the remaining set is empty or the
-timeout expires (cb, registered in init()) a notify completion
-is sent to the client.
-
-Watch Lifecycle
----------------
-A watch may be in one of 5 states:
-
-1. Non existent.
-2. On disk, but not registered with an object context.
-3. Connected
-4. Disconnected, callback registered with timer
-5. Disconnected, callback in queue for scrub or is_degraded
-
-Case 2 occurs between when an OSD goes active and the ObjectContext
-for an object with watchers is loaded into memory due to an access.
-During Case 2, no state is registered for the watch.  Case 2
-transitions to Case 4 in PrimaryLogPG::populate_obc_watchers() during
-PrimaryLogPG::find_object_context.  Case 1 becomes case 3 via
-OSD::do_osd_op_effects due to a watch operation.  Case 4,5 become case
-3 in the same way. Case 3 becomes case 4 when the connection resets
-on a watcher's session.
-
-Cases 4&5 can use some explanation.  Normally, when a Watch enters Case
-4, a callback is registered with the OSDService::watch_timer to be
-called at timeout expiration.  At the time that the callback is
-called, however, the pg might be in a state where it cannot write
-to the object in order to remove the watch (i.e., during a scrub
-or while the object is degraded).  In that case, we use
-Watch::get_delayed_cb() to generate another Context for use from
-the callbacks_for_degraded_object and Scrubber::callbacks lists.
-In either case, Watch::unregister_cb() does the right thing
-(SafeTimer::cancel_event() is harmless for contexts not registered
-with the timer).
-
-Notify Lifecycle
-----------------
-The notify timeout is simpler: a timeout callback is registered when
-the notify is init()'d.  If all watchers ack notifies before the
-timeout occurs, the timeout is canceled and the client is notified
-of the notify completion.  Otherwise, the timeout fires, the Notify
-object pings each Watch via cancel_notify to remove itself, and
-sends the notify completion to the client early.